1. Field of the Invention
The present invention relates to a solid-state image pickup device, a method for manufacturing the same, and an electronic apparatus.
2. Description of the Related Art
Electronic apparatuses, such as digital video cameras and camera-equipped cellular phones, include solid-state image pickup devices.
The solid-state image pickup device has an image-pick up region including a plurality of pixels on a semiconductor substrate. Each pixel has a photoelectric conversion portion. The photoelectric conversion portion, which may be a photodiode, generates a signal charge by receiving light coming through an external optical system and converting the light to electricity.
A CMOS (complementary metal oxide semiconductor) image sensor can be used as the solid-state image pickup device.
The CMOS image sensor includes pixels, each including the photoelectric conversion portion and a set of transistors. The set of transistors acts as pixel transistors that read the signal charge generated from the photoelectric conversion portion and output the signal charge to a signal line as an electric signal. For example, pixel transistors including four transistors: a transfer transistor, a reset transistor, an amplification transistor and a selection transistor are provided on the surface of the semiconductor substrate. The transistors of the pixel transistor sets are electrically connected to conductor lines disposed on the surface of the semiconductor substrate.
The CMOS image sensor is generally of front emission type in which the photoelectric conversion portion receives light entering through the front side of the semiconductor substrate having conductor lines and pixel transistors. In the surface emission type, unfortunately, conductor lines are disposed in a plurality of layers in the path through which light enters the photoelectric conversion portion. Therefore the conductor lines in the plurality of layers may reduce the light use efficiency to reduce the sensitivity. Accordingly, a rear emission type is proposed which receives light entering through the rear side of the semiconductor substrate opposite the front side having conductor lines and pixel transistors (for example, Japanese Unexamined Patent Application Publication No. 2003-31785).
It is advantageous that the CMOS image sensor is more miniaturized and achieves high definition. In order to reduce the pixel size for miniaturization of the CMOS image sensor, a pixel structure is proposed in which a plurality of photoelectric conversion portions share the pixel transistors. For example, a technique is proposed in which a single pixel transistor set is provided for two or four photoelectric conversion portions (for example, Japanese Unexamined Patent Application Publication Nos. 2004-172950, 2006-157953 and 2006-54276).
In such a technique, a pixel isolation member is formed in the substrate to separate pixels from each other. For example, a shallow-trench oxide isolation (STI) structure is provided as the pixel isolation member in the semiconductor substrate. Alternatively, an impurity diffusion region, such as a p-well, is provided as the pixel isolation member in the semiconductor substrate (for example, Japanese Unexamined Patent Application Publication No. 2006-93319 and 2007-53250).